1. Field of the Invention
The present invention relates to a cylinder head assembly for use in a internal combustion engine, and particularly, to the structure of a rocker-arm shaft cap of the cylinder head assembly.
2. Description of the Related Art
As a valve drive mechanism, a variable valve timing mechanism is well-known in which two kinds of cams, i.e., a cam for high rotational speed and a cam for low rotational speed, having different profiles are coupled with a camshaft, and these two cams are hydraulically switched from one to the other such that valves are driven by the low rotational speed type cam when the engine is driven within a low rotational speed range or by the high rotational speed type cam when the engine is driven within a high rotational speed range. Such switching is done for the purpose of improving the output torque of the engine within a high rotational speed range, and at the same time, maintaining drivability thereof within the low rotational speed range, i.e., ensuring high torque within the low rotational speed range. Various structures have been proposed as the structure of such a variable valve timing mechanism, and there has been proposed a structure for a variable valve timing mechanism as shown in FIGS. 12 and 13, which is disclosed in U.S. patent application Ser. No. 725,605 filed Jul. 3, 1991.
The variable valve timing mechanism 1 shown in FIGS. 12 and 13 includes two valves 2 and 3, T-shaped rocker arm 5 for opening and closing valves 2 and 3, low rotational speed type rocker arm 6, a high rotational speed type rocker arm 7, pistons 8 and 9 included in T-shaped rocker arm 5, for selectively coupling low rotational speed type rocker arm 6 or high rotational speed type rocker arm 7 with T-shaped rocker arm 5, and low and high rotational speed type cams 14 and 15 formed on cam shaft 13.
T-shaped rocker arm 5 has base end 5a, and rocker-arm shafts 4 and 4' are integrally formed so as to project from both sides of base end 5a. Rocker-arm shafts 4 and 4' are rotatably supported by rocker shaft journals 12 and 12'. Oil pressure P is applied to piston 8 through oil path 4a provided in journal 12' and shaft 4', and oil pressure P is also applied to piston 9 through oil path 4a provided in journal 12 and shaft 4.
Low rotational speed type rocker arm 6 and high rotational speed type rocker arm 7 respectively have shaft holes 6a and 7a at their base ends, and rocker-arm shafts 4' and 4 of rocker arm 5 are respectively engaged in the shaft holes 6a and 7a, thereby to support low and high rotational speed type rocker arms 6 and 7 such that they can swing. Roller bearings 10 and 11 are provided at the distal ends of low and high rotational speed type rocker arms 6 and 7, respectively. These roller bearings 10 and 11 can rotate when brought into contact with respective low and high rotational speed type cams 14 and 15.
When the engine is driven within a low rotational speed range, piston 8 is forced out of a piston hole by the spring force of spring 17', and the top of piston 8 is inserted into piston hole 6b of low rotational speed type rocker arm 6, as is shown in FIG. 13. In this case, low rotational speed type rocker arm 6 and T-shaped rocker arm 5 are coupled with each other, thereby to serve as one single unit, so that low rotational speed type cam 14 drives valves 2 and 3 through low rotational speed type rocker arm 6 and T-shaped rocker arm 5.
On the other hand, piston 9 is maintained in a piston hole by the spring force of spring 17, so that high rotational speed type rocker arm 7 can freely swing, when released from being coupled with T-shaped rocker arm 5, as is shown in FIG. 13.
When the engine runs in a high rotational speed range, oil pressure P is supplied to oil path 4a (see FIG. 13) of rocker-arm shaft 4, thereby to force piston 9 out of its piston hole against the spring force of spring 17, so that the top of piston 9 returns back and is inserted into piston hole 7b of high rotational speed type rocker arm 7. In this case, high rotational speed type rocker arm 7 and T-shaped rocker arm 5 are coupled with each other, thereby to serve as a single unit, so that high rotational speed type cam 15 drives valves 2 and 3 through high rotational speed type rocker arm 7 and T-shaped rocker arm 5.
In an engine using a valve drive mechanism in which rocker-arm shafts 4 and 4' swing together with T-shaped rocker arm 5, as has been explained above, it is necessary to adopt a structure which functions as bearings at the portion of the cylinder head where the rocker-arm shafts 4 and 4' are supported, i.e., it is necessary to adopt a method for supporting the rocker-arm shafts, in which the rocker-arm shafts may be allowed to swing. In a conventional method, block type rocker shaft holder 20 as shown in FIG. 14, cap type rocker shaft holder 21 as shown in FIG. 15, or the like may be used to support the rocker-arm shafts.
However, in use of conventional rocker shaft holders 20 or 21, rocker-arm shafts 4 and 4' are first respectively engaged with journal portions 20a or 21a of two holders, and the holders are then incorporated into cylinder heads, one after another. Therefore, a conventional cylinder head assembly results in problems in that much time and labor are required for the installation of rocker-arm shafts, and the bearing portions of the cylinder head assembly exhibit only low rigidity.